The present invention relates to a light guide plate used in liquid crystal display devices and the like, a light guide plate unit using the light guide plate, and a planar lighting device using the light guide plate and the light guide plate unit.
Liquid crystal display devices use a backlight unit for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using a light guide plate for diffusing light emitted by an illuminating light source to irradiate the liquid crystal display panel and optical parts such as a prism sheet and a diffusion sheet for rendering the light emitted from the light guide plate uniform.
Currently, large liquid crystal televisions predominantly use a so-called direct illumination type backlight unit comprising a light guide plate disposed immediately above the illuminating light source (see, for example, JP 05-4133 U). This type of backlight unit comprises a plurality of cold cathode tubes serving as a light source provided behind the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to ensure uniform light amount distribution and necessary brightness.
To achieve a uniform light amount distribution with a direct illumination type backlight unit, however, a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel is required, making further reduction of thickness difficult with the direct illumination type backlight unit.
Small liquid crystal monitors used in computers and the like, on the other hand, employ a backlight unit of a so-called side light type using a flat light guide plate to achieve smaller, thinner designs. Also proposed is a thin backlight unit of a type using a light guide plate of which the thickness decreases with the increasing distance from the light source such as, for example, a tandem type (see, for example, JP 02-208631 A, JP 11-288611 A, and JP 2001-312916 A).